Thermal ink-jet printers offer a low-cost, high quality, and comparatively noise-free option to other types of printers commonly used with computers. Such printers employ a resistor element in a chamber provided with an egress for ink to enter from a plenum The plenum is connected to a reservoir for storing the ink A plurality of such resistor elements are arranged in a particular pattern, called a primitive, in a printhead. Each resistor element is associated with a nozzle in a nozzle plate, through which ink is expelled toward a print medium. The entire assembly of printhead and reservoir comprises an ink-jet pen.
In operation, each resistor element is connected via a conductive trace to a microprocessor, where current-carrying signals cause one or more selected elements to heat up. The heating creates a bubble of ink in the chamber, which is expelled through the nozzle toward the print medium. In this way, firing of a plurality of such resistor elements in a particular order in a given primitive forms alphanumeric characters, performs area-fill, and provides other print capabilities on the medium.
A problem with inks used in such thermal ink-jet printers is that the repeated heating of the resistor element over several hundreds of thousands or over millions of firings can cause breakdown of the ink, especially the organic components, such as the dye, with consequent fouling of the surface of the resistor element. This process has been termed "kogation", which is defined as the build-up of residue (koga) on the resistor surface. The build-up of residue degrades pen performance by reducing the volume of delivered ink over pen lifetime.
In the anionic dyes (sulfonate or carboxylate) commonly employed in aqueous inks used in thermal ink-jet printing, sodium is generally the counter-ion used. While dyes containing sodium counter-ions generally provide good print quality, in some inks, sodium counter-ions have been found to contribute to the kogation problem.
Various ink compositions and processes have been developed in an effort to reduce kogation. One solution has been to partially or totally replace the counter-ion on the dye with a replacement counter-ion, such as lithium and tetramethylammonium. In other instances, oxo anions, such as phosphates, have been shown to reduce kogation, at least in some inks.
Yet, other inks experience a decrease in drop volume over the life of the pen. A change in drop volume may indicate the formation of resistor residues, and hence the presence of kogation.
Originally, the problem was associated with a build-up of organic residue visible upon microscopic examination of the resistor pads with pen lifetime. Thus, the coated resistors were less efficient at heat transfer than those that were not coated. However, there are cases where decreases of drop volume with pen lifetime occur in the absence of any visible residue accumulation on the resistor pads. Clearly, in both cases interference in bubble generation occurs, but a common explanation has remained elusive. It is believed that both the surface chemistry and/or solution chemistry of the ink could be the cause of the problem.
As an example, an ink containing 1.01% N,N-dimethyl-N-(Z-9-octadecenyl)-N-amine oxide (OOAO), 2.24% SURFYNOL 465 (SURFYNOL is a trademark of Air Products and Chemicals, Inc.), 0.28% sodium alginate (low viscosity from Sigma), 9% 1,5-pentanediol, 0.3% UCARCIDE (from Union Carbide), one of the following dyes: 1.5% Acid Red 52-Na, 1.3% Acid Blue 9-Na, Direct Yellow 86-TMA, or 0.5% Acid Yellow 23-TMA, where TMA is tetramethylammonium cation in place of Na cation, and the balance water, shows a decrease of deliverable drop volume with pen lifetime at 45% over the turn-on-energy (TOE) of the pen for both the cyan and yellow inks. The magenta ink appears to be unaffected. This energy is at the upper operating energy of the pen and printer, and while the number of pen and printer combinations that might experience this problem is small, a potential problem exists.
In an attempt to relieve the problem, (NH.sub.4).sub.2 HPO.sub.4 was tried at the level of 0.5%. Use of this reagent was unsuccessful in this system. Although the yellow ink kogation improved, the cyan ink still experienced a 20 to 30% deviation in drop volume over pen lifetime. Severe decap and crusting performance problems occur at this level of (NH.sub.4).sub.2 HPO.sub.4 for both inks, and thus this reagent, which has successfully been used with other inks (see, U.S. patent application Ser. No. 07/428,282, filed Oct. 27, 1989, and assigned to the same assignee as the present application), could not be used with these inks.
The need remains for the development of inks having reduced kogation, and hence resulting in longer pen life, using low cost chemicals with minimal additional processing.